This invention pertains to an apparatus and method for forming plugs in a wellbore, such as in oil and gas recovery operations.
It is often necessary to form a plug in a wellbore that penetrates a subterranean earth formation in an oil and gas recovery operation. Such plugs are used for many reasons. For example, the formation surrounding the wellbore, with its fractures, large pores, and other openings, often will be so porous that it absorbs a great deal of any type of fluid that is introduced into the wellbore. To prevent this, a cement slurry is passed from the ground surface, through tubing and into the lower portion of the wellbore where it accumulates to allow some of it to penetrate the formation and fill the fractures, pores and openings. After the cement hardens, some, or all, of the hardened cement remaining in the wellbore is drilled out so that other fluids can be passed through the bore without the absorption problem.
U.S. Pat. No. 6,772,835 discloses a work string including tubing and a downhole tool connected to the tubing for facilitating the introduction of the cement slurry and allowing some of the tubing to be recovered. The tool includes a sacrificial tailpipe portion that can be decoupled from the remaining portion of the tool to allow the latter portion of the tool, as well as the tubing above the tool, to be recovered after the cement plug is formed. The disclosure of this patent is incorporated by reference.
However, there is a certain limit to the amount of slurry a formation can withstand before it collapses. Therefore, in relatively large installations, an initial charge of cement slurry is introduced into the well through the tool described above, with the volume of the charge, and therefore the height of the wellbore that is filled with cement, being less than optimum so as to not damage the formation. Then, the remaining portion of the tool and the tubing above are withdrawn in the manner disclosed in the above patent. After the cement hardens, the process has to be repeated with one or more additional charges of cement slurry until the plug extends to a desired height in the wellbore. This, of course, considerably adds to the cost of the operation.
Therefore, what is needed is a system and method for forming plugs in a wellbore that overcome the above problem.